1. Field of the Invention
The present invention is directed to pathogen resistance in plants. More particularly, the invention is directed to identification and use of a gene that provides resistance to late blight disease. Even more particularly, the invention is directed to a Solanum bulbocastanum late blight resistance gene, nucleic acid molecules encoding polypeptides which confer resistance to late blight, and methods of using the gene, including expression in plant cells to confer or enhance a plant's resistance to late blight.
2. Description of the Art
On a worldwide basis, late blight, caused by the fungus Phytophthora infestans, is the most important of potato diseases. Worldwide losses due to potato late blight are estimated to be about $3 billion annually. Conservatively, P. infestans costs the potato industry in the United States $200 to $400 million annually.
Currently, late blight is controlled by application of fungicides. The cost of chemical control in the U.S., now applied in essentially all potato producing regions, is approximately $100–$200 per acre. Given that approximately 1.2 million acres are planted to potatoes annually in the U.S., the control costs alone are significant. In addition, in many years storage losses due to this pathogen are in the same range as the cost of control.
In the U.S., the recent migration from Mexico of highly aggressive and virulent new forms of P. infestans poses a serious threat to all potato producing regions. In particular, the presence of A2 mating type and fungicide resistant forms in field populations of the fungus limits producers' options in control practices.
P. infestans also causes late blight in other crops, including tomato, eggplant, and other solanaceous species. The new, aggressive strains of P. infestans also represent a serious threat to commercial tomato production.
Identification of a late blight resistance gene and development of transgenic plants resistant to P. infestans, is important goal in plant research to reduce crop losses and to reduce the need for fungicide application and costs of chemical control.
A wide variety of genetic loci that confer resistance to pathogens have been identified in plant species. These resistance loci often encode dominant resistance genes, or R genes. The R genes confer either vertical race-specific or horizontal nonspecific resistance to a pathogen (Plank, 1968). Vertical resistance is based upon an induced hypersensitive response in which the pathogen infection is contained by localized host cell death at infection sites. The mechanism for vertical resistance has been proposed to involve activation of the cell death response when a specific plant receptor (the R gene product) interacts with an elicitor produced by a corresponding Avr gene in the invading pathogen (Flor, 1971). Pathogen races are defined by distinct Avr gene profiles and resistance results from the interaction between specific R gene and Avr gene products (the gene for gene interaction).
In contrast to vertical resistance, horizontal resistance generally involves multiple plant genes and provides a general, stable, pathogen resistance in a race-nonspecific manner. Horizontal resistance is not correlated with the hypersensitive response, involving instead limiting pathogen spread in the host. Solanum bulbocastanum contains a dominant R gene locus which confers horizontal resistance to P. infestans when introgressed into the cultivated potato (Naess et al., 2000; Naess et al., 2001).
Map-based cloning has been employed to identify a variety of R genes from crop plants (Ballvora et al., 2002; Brueggeman et al., 2002; Dixon et al., 1996; Feuillet et al., 1997; Lagudah et al., 1997; Ori et al., 1997; Yoshimura et al., 1998).